The present invention relates to a current-carrying lead for supplying an electric current to superconducting magnets and like devices.
An electric current is supplied to superconducting magnets and other superconducting devices by means of current-carrying leads provided between the source of current supply and the device. Conventionally, such current-carrying leads are made of copper conductors in rod or pipe form. However, copper has both electrical conducting (non-superconducting) and thermal conducting properties. Because of these properties, the heat generated by the copper conductor when a current is applied and the heat transmitted from the terminal on the hot side are conducted to liquid helium, liquid nitrogen or other cryogenic media (cryogens) in which the superconducting magnet is submerged, thereby causing such cryogens to evaporate.
To minimize the heat conduction to liquid helium, several methods have been proposed for use with prior art current-carrying leads and one common approach is depicted in FIG. 1. There, spiral cooling fins 110 are provided on the surface of the lead which is accommodated in a stainless steel sheath 120. Notwithstanding this cooling structure, the helium gas 130 is evaporated by the heat of conduction drawn into the lead 100 from a terminal 140 connected to the superconducting magnet and the heat generated by the electrical current supplied to the magnet through the cooled lead and a terminal 150. In an attempt to further reduce the conduction of heat to the liquid helium, further testing and research is underway, with emphasis being placed on the effort to improve the construction of current-carrying leads. However, it has been generally believed that a heat conduction of 1 watt per 1,000 amps of applied current is unavoidable in the state of the art.
Current-carrying leads for use with superconducting magnets and like devices are also available in "detachable" form consisting of a socket. A plug and a prior art product of this type is illustrated in FIG. 2. A plug 21 made of copper is coupled to a terminal on the hot side 24 to a power source (not shown) via a normal conducting current-carrying lead 23 made of copper in rod, pipe or busbar form. A socket 22 is provided at a terminal on the cold side 26 to a superconducting magnet or like device (not shown). The plug 21 is inserted into the socket 22 for conducting current or is detached from the socket to cut off the current supply. The socket 22 is made of brass or copper and is furnished in its interior with one or more beryllium-copper, multi-face contactors 25, depending upon the current-carrying capability of the lead. The spring properties of the contactors are utilized to minimize the contact resistance between the plug 21 and socket 22.
The socket and plug in the above-described detachable current-carrying lead are made of copper conductors or brass (in the case of the socket) which are both good electrical conductors and good heat conductors as well. Accordingly, a Joule loss will occur as a result of a voltage drop that takes place in the material itself when current is applied and which also takes place due to the contact resistance between the socket and plug. The heat generated by this Joule loss is transmitted into the liquid helium, liquid nitrogen or other cryogenic media in which the superconducting magnet is submerged, and the heated cryogens will evaporate.
In order to ensure maximum current-carrying capability of the lead, the normal conducting metal must have a sufficient cross-sectional area. However, a greater area will result in increased heat condition from the hot area to the cryogenic medium. Research has been focused on methods to minimize this heat conduction to the cryogenic media. It has been found to date that a voltage drop of about 13 mV per 1,800 A of applied current is unavoidable between plug and socket. If a pair of leads having opposite polarities are used, the overall heat conduction is about 1.800 A.times.0.013 V.times.2=46.8 W. Liquid helium used as a cryogen will evaporate in an amount of 1.4 L/h per heat conduction of 1 W and as much as 65.52 L/h of liquid helium will volatilize under heat conduction of 46.8 W.